The present invention relates to a novel key board switch unit with illumination suitable for use even in a dark place.
Many modern electronic appliances, e.g. electronic pocketable calculators and the like, are provided with a key board switch unit for operating the instrument. There are growing occasions of operating such a key board switch unit in a relatively dark place such as in automobiles at night so that many of the key board switch units are provided with a means for illumination.
The most simple method for illuminating a key board switch unit is the illumination of the board from above with a suitable lamp. Alternatively it is also conventionally practiced that the surface of the key top is coated with a phosphorescence or fluorescence material which emits visible light in a dark place to assist the recognition of the key board by the operator. Further, the pushing areas of the key top are made with a transparent or translucent material and illuminated with a suitable light source installed just below the pushing area of the key top.
The first method of illumination from above is the simplest in principle and sufficient lighting is readily obtained. This method is, however, impracticable when a very thin design of the key board switch unit is desired. In addition, complete shielding of stray light is sometimes very difficult in such a top illumination bringing about troubles such as glaringness when the key board switch unit is used in the room of an automobile or in a photographic dark room.
The second method of the use of a phosphorescent material is defective in the insufficient intensity of light emitted from the phosphorescent material so that it is not always suitable as an illuminating means of a key board switch unit.
The third method of the illumination from below the pushing areas of the key top also has a problem when a compact design of the unit is desired and uniform illumination of all of the pushing areas of the key top is obtained only with a considerable number of the light sources so that the method is not applicable when the electronic appliance having the key board switch unit is very thin as pocketable electronic calculators.
Thus, it has been an eager demand to develop a key board swtich unit with illumination free from the above described problems in the prior art and many attempts have been made therefor but without success.